The invention relates to a converter comprising switching elements for chopping a direct voltage, in which turn-on phases of the switching elements alternate with one another, and comprising a circuit arrangement with resonant circuit elements processing the chopped direct voltage and serving for the supply of an output voltage.
Converters of this type generally represent switched-mode power supplies which are used for DC power supply of a load connected to the output of the switched-mode power supply. In such switched-mode power supplies, an AC voltage at the input is rectified so as to obtain a DC voltage at the converter input. However, the invention also relates to converters whose inputs receive a direct voltage directly from a DC voltage source. The DC input voltage is chopped by means of a bridge circuit consisting of switching elements. The chopped DC voltage is applied to a circuit arrangement comprising resonant circuit elements, i.e. inductive and capacitive reactances so that an approximately sinusoidal alternating current flows in the circuit arrangement. There should be at least an inductive and at least a capacitive resonant circuit element. A load may be connected at the output of the circuit arrangement and hence at the output of the converter. By adapting the switching frequency, an adaptation to variations of the load and input voltage fluctuations is effected. Converters with resonant circuit elements, i.e. resonance converters, allow operation at high switching frequencies of the switching elements and thus realize relatively small-volume and lightweight devices as regards possible power output. When resonance converters are used, particularly also zero-voltage switching (ZVS) is possible with a small number of circuit components. In this connection, ZVS refers to turning on switching elements (rendering them conducting) at a minimal switching element voltage, preferably near zero volt. In ZVS, the circuit arrangement with the resonant circuit elements has an inductive input impedance as considered from the side of the switching elements. In the case of ZVS, MOSFET transistors are commonly used as switching elements.
To allow ZVS, dead-time phases must be provided in which all converter-switching elements are turned off (i.e. rendered non-conducting). As is known, the length of the dead-time phases is adapted to different fields of use (see, for example, STR-Z4000 series of the firm of Allegro-Sanken, data sheet November 1995, or the Controller-IC MC 34067 of Motorola, data sheet 1996xe2x80x94both ICs are used for switching element control). In the IC of the firm of Allegro-Sanken, this is done, for example, by means of an external resistor, and in the IC of Motorola this is done by fixing a resistance-capacitance combination. ZVS in the given converter structures can of course only be guaranteed for limited ranges of operation. Outside these ranges of operation, for example, in the case of large input voltage differences or large load differences to be processed, essential modifications of the converter are necessary so that particularly its manufacturing costs will increase to an undesirably high extent. In the case of erroneous adaptation of the dead-time phases and the resultant disabled ZVS, the switching losses will increase, which in extreme cases may lead to destruction of the switching elements.
It is an object of the invention to improve the converter of the type described in the opening paragraph to such an extent that ZVS can be ensured in a wider range of operation of the converter and with possibly inexpensive converter modifications.
This object is achieved by an automatic adaptation of the length of the dead-time phases which are present between two consecutive turn-on phases and during which the switching elements are turned off.
The converter according to the invention allows ZVS within a wider range of operation. Switching losses occurring when turning on the switching elements can be minimized. Moreover, it can be adapted to various fields of applications while using a small number of components. Moreover, the invention allows a simple adaptation of the converter in the case of changes of the output power to be supplied, in the case of changing to other switching element types (for example, to another MOSFET transistor type with other parasitic capacitances) or in the case of using other converter components, or a change of tolerance ranges in the converter components used. The required dead-time adaptation can be achieved by an appropriate and easy-to-realize adaptation of the control circuits used for controlling the switching elements, i.e. particularly by means of appropriately programmed ICs. The converter output supplies particularly a direct voltage controlled to a fixed value. However, the use of the converter for supplying a constant direct current is also possible. Basically, the converter could also supply an alternating voltage or an alternating current; for this case, a rectifier arrangement at the output of the converter would not be required.
To realize the automatic adaptation of the length of the dead-time phases, two variants are proposed. In a first variant, a first measuring device is provided for measuring a voltage decreasing across one of the switching elements and a first comparison device is provided for generating a comparison signal causing the switching elements to be turned on when the switching element voltage reaches a first threshold value during a dead-time phase. In the second variant, a second measuring device is provided for measuring the variation with respect to time of a voltage decreasing across one of the switching elements, and a second comparison device is provided for generating a comparison signal when the variation with respect to time of the switching element voltage falls below a second threshold value during a dead-time phase, said comparison signal causing the switching element to be turned on.
The first variant requires a more elaborate measuring device than the second. In contrast, however, the first variant allows a more precise adaptation of the length of the dead-time phases than the second.
In a further embodiment of the converter according to the invention, a timer is provided for predetermining the maximum length of the dead-time phases. This is an additional safety measure which also prevents the maximum length of the dead-time phases from being exceeded in the case of erroneous adaptations.
The invention also relates to a control circuit constructed particularly as an integrated circuit (IC) for controlling at least one of the switching elements of the converter according to the invention, which control circuit is provided for supplying control signals effecting an automatic adaptation of the length of the dead-time phases which are between two consecutive turn-on phases and during which the switching elements are turned off.